Electrical contact device



Aug-4,1953 w. R. JACK ETAL 2,647,975

ELECTRICAL CONTACT DEVICE Filed Dec. 19, 1951 IN V EN TORS Y WILL/AM R. JACK I yv3 BY Y -E Axum/L4 A T TOR/V6 Y Patented Aug. 4, 1953 ELECTRICAL CONTACT DEVICE William R. Jack and Lysle D. Cahill, Chagrin Falls, Ohio; said Cahill assignor to said Jack Application December 19, 1951, Serial No. 262,432

2 Claims.

This invention relates in general to electrical contacts and more particularly to sliding contacts. In the United States patent to William R. Jack et al., No. 2,537,671, Variable Resistance Device, January 9, 1951, is disclosed a permanent wear resisting variable electrical resistor including a new and novel resistance material structure to be engaged by a movable contactor. In this patent is also disclosed a movable contactor in the form of a mercury globule as the element that actually contacts the resistance structure. The choice of a mercury globule is an excellent one for a number of reasons, among which are its frictionless and non-abrasive characteristics as it engages the resistance structure, especially in view of the highly exacting requirements in a variable resistor of this general character. In one embodiment of this patent, the mercury contactor is shown in the form of a mercury ball with an adjustable spring actuated piston as a means for resiliently forcing the mercury ball into constant contact with the surface of the resistance structure.

In these and other electrical devices employing mercury ball contactors it is highly important to have means for exerting constant resilient pressure of the mercury ball on the engaged surface to compensate for possible surface irregularities. At the same time, it is important that such pressure remain sufficient to accomplish this and not excessive enough to shatter, or otherwise displace, the liquid mercury ball from its proper location in its holder in the movable contactor arm. Such contingencies have been found to exist when pistons are employed as a pressure means for the mercury ball, regardless of the means for actuating the piston, such as springs, magnets, electromagnets, and the like. This is because of the always existing friction attending the action of the piston and the difficulty, if not the practical impossibility, of proper adjustment and regulation of the piston actuating means.

One of the primary objects of our invention is to provide an improved mercury ball contact holder that may be rigidly carried by the contact arm and so inherently designed that will insure that the mercury ball itself will inherently exert a continuous resilient pressure against the resistance surface it engages and be safeguarded against fracture, or other displacement all to the elimination of any such friction members as pistons or such piston actuating means as springs, magnets, electromagnets, and the like.

Applying the principle that there are inherent in a globule of mercury physical forces that cause the globule to assume a spherical shape, we propose to provide a single piece holder with a hollow bore to receive the mercury globule. The outer end portion of the bore i not semispherical or cylindrical, but, on the contrary, is generally conical by comparison. The globule in its effort to assume a spherical form, follows the path of least resistance so that it does not completely fill the cone, but flows toward the large end of the cone, so that the portion that protrudes from the cone is generally semi-spherical. Moreover, this force inherent in the mercury globule is resilient. Thus the contact pressure is resilient without extraneous resilient or electrically operated piston means, and the mercury globule is safeguarded against fracture which may otherwise occur by use of such pistons, or in fact, by the use of a semispherical cup shaped holder.

Moreover, there is provided, in communication with the conical bore, a restricted bore, which is insufficient in size to permit liquid mercury flow therethrough. Its purpose is to provide a vent to allow the escape of air, liquid, or the like from behind the liquid mercury globule that might otherwise be trapped and thus exert an undesired outward pressure on the liquid mercury globule. With the foregoing and other objects in view, the invention resides in the combination of parts and in the details of construction hereinafter set forth in the following specification and appended claims, certain embodiments thereof being illustrated in the accompanying drawings, in which:

Figure l is a view in longitudinal section taken through a variable resistance device, also showing in vertical and longitudinal section, our electrical contact device assembled therein;

Figure 2 is an end view of the variable resistance device, with the cover plate partly broken away to partially show the electrical contact device in elevation installed therein;

Figure 3 is an enlarged fragmentary view in section taken through a portion of the electrical contact device and of the electrical resistance structure of the variable resistance device.

Referring more particularly to the drawings, we have shown, by way of example, merely one use to which our new and novel electrical contact device may put, by showing it to be incorporated in a variable resistance device, and more specifically such a variable resistance device which is the subject matter of the aforesaid patent. This is a good example of the utility of our improved electrical contact device, because of the rigid requirements of the electrical contact devices employed therein. 1 I

As shown in the drawings, the variable resistance device may comprise a cylindrical casing I having a cover plate 2 removably secured to the casing by screw bolts 3. Suitably positioned within the casing I is a cylindrical potentiometer ring comprising a base 4 and a resistance material structure generally indicated at 5, on the inside surface of the base. The casing also has an externally screw threaded sleeve 5 for the proper mounting of the unit wherever desired and shaft i is mounted to be rotatable therein. Shaft l carries a flange 8 that carries a resilient arm 5 provided with a ball l0 urged against contact elemerit ll of terminal I2. Terminal is has a resilient member It in contact with the petentiorneter ring. Flange 8 also carries a contactor, assembly generally indicated at IS the actual contact member of which is urged into constant contact with the inner surface of the resistance materiai tpncs ture on the inside of thepotentiometer ring.

While e base 4 m be ce amic, gla s. o other material, for purposes of illustration it will be referred to as a glass cylinder. The potentiometer ring is shown as having a resistance material structure'5 that is virtually an integral part of the base 4. This may be accomplished, for instance, by evaporating a resistance metal such as platinum on to the inner surface of the glass ring. In order to evaporate the platinum on to and into the glass the platinum is heated to a temperature above its melting point in a vacuum. Such evaporated metal processes are readily controllable to insure imbedding of the platinum into the glass bowl and provide a coating whose thickness and width is also readily controllable for any desired pattern of varying thickness or width or both.

The contactor assembly has been generally referred to at 5, which, in reality, may be a rigid, non-resilient sleeve that is rigidly, and non-resiliently connected to flange 8 that is carried by rotatably mounted rigid shaft 2'. As shown in Figure 1, this sleeve is adapted to rigidly, and non-resiliently, receive and hold a hollow contact holder 16 that is shown in enlarged detail in Figure 3. This hollow holder is shown as having three distinct, but intercommunicating hollow bores l8, l9 and 2H and the outer end of the holder i6 is shown as having its outside diameter bevelled at 2| to taper in decreasing thickness. The outer hollow bore '20 is generally conical, or frustro-conical from the outer end of holder it to the straight communicating relatively restricted bore [9 of some suitable length, such as that shown in Figure 3, until it reaches the point of communication with the relatively larger hollow bore [8. The actual contact member is a globule of liquid mercury [7, shown in Figure 3 as filling the conical bore 2!] and partially protruding outwardly past the outer end of the contact holder [5.

As stated before, liquid mercury has the inherent physical characteristics that cause a liquid mercury globule to exercise a force to assume a spherical form. If a globule were deposited in a semi-spherical cup and the globule were of a sufiicient size to completely fill the cup, the globule would protrude from the cup to present a semi-spherical portion outside of the cup as well. Such a device, however, would not make a satisfactory contact device, and this is true whether the cup is held rigidly or provided with resilient means for urging the mercury globule into engagement With the contact surface. This is because of the danger of fracture, or other displacement, of the globule from its holder during its engagement with the contact surface, whether by movement of the contact holder in normal use or inadvertent shock occasioned by the device into which the contact device assembly is incorporated. The cup being full, there is no place for the mercury to go but out of the cup. This danger is a contingency whether rigid or flexible pressure is applied externally to urge the mercury into contact with the contact surface. This is especially true when there is no provision made for the escape of air that may be trapped in the cup back of the liquid mercury.

Accordingly, we propose to provide a holder as shown in Figure 3. We pour, Or insert, a globule of liquid mercury I'l into the conical bore 2Q at the outer end of holder i6 and of a quantityor size, sufficient to insure that a desired portion protrudes outwardly from the cone. The globule, due to its inherent forces causin it to attempt to assume a spherical shape, does not completely fill the cone but follows the path of least resistance and bulges out of the open end of the cone so that the shape of the portion of the globule that extends out. of the cone is generally semi-spherical, as compared to the frustro-conical portion of the globule that remains in the holder. This conical bore 2!) is of utmost importance. It enables the liquid mercury globule to impart a resilient force against the contact surface 5, Without the aid of external spring or other mechanical devices, or electrical or electrical field forces, and their attending disadvantages. The cone cup causes the liquid mercury to exercise a greater force, because of its confinement, than a semispherical cup. The cone 20 permits the liquid mercury to partially retreat thereinto and provide a shock absorbing medium with respect to pressure and shook. As in example of materials, the holder it may be steel, or other material which will not be adhered to, or be wetted by liquid mercury. Thus the flow of the mercury is regulated strictly by pressures and surface tension. In inherently forcing itself out of the cone, the mercury also inherently exerts a resilient shock absorbed contact pressure against the contact surface. It, therefore, follows any and all inaccuracies, or surface irregularities of the Contact surface Without any loss of contact pressure. The engagement of the mercury With the contact surface is frictionless, without Wear on thecontact surface and to the elimination of any hysterisis or any tendency of dragging or sticking. In fact, the contact is a spring contact Without the use of any spring. There are no control devices necessary or employed.

If, instead or the use ofliquid mercury, a wetting liquid were used, there would be the force of adhesion of the liquid to the Wall of the holder, as well as the force from the surface tension of the liquid itself. The resultant force would be such as to draw liquid toward the small end of the cone. However, in our use of liquid mercury, the liquid, that does not wet the cone, we eliminate the force of adhesion of the liquid to the cone wall. The resulting force of surface tension of the mercury is smaller at the larger end of the cone than at the smaller end of the cone. Hence, it follows that our globule of non-wetting liquid mercury is resiliently driven to the large end of the cone.

The cone has its inside wall straight and uninterrupted and is inherently a shock absorbin medium when the mercury is subjected to pressure or shock and no retaining lip is necessary, nor employed.

We claim:

1. In combination in an electrical assembly, a housing, a contact surface, an electrical contact device including a carrying arm mounted for movement in said housing, a rigid holder member rigidly carried by said arm, said holder having a longitudinal hollow substantially conical bore extending continuously to become lar at its radially outward end at the outer end Of the holder, a globule of liquid mercury in said conical bore in said holder and, by reason of its inherent physical characteristics in its efiort to assume a spherical shape, being inherently adapted to exert a constant resilient contact pressure on said contact surface, said holder having a vent therein for the escape of air from behind the mercury in said holder.

2. In combination, a variable resistance having a housing and a substantially flat contact surface, an electrical contact device including a carrying arm mounted for rotation in said housing, a rigid holder member rigidly carried by said arm, said holder having a longitudinal hollow bore the outer end portion of which is substantially conical continuously and uninterruptedly to the outer end of the holder, a globule of liquid mercury in said conical bore in said holder and, by reason of its inherent physical characteristics in its effort to assume a spherical shape, being inherently adapted to exert a constant resilient contact pressure on said contact surface, said holder having a relatively restricted longitudinally inwardly extending passage in communication with said conical bore to comprise a vent for the escape of air, or liquid other than mercury, that may be trapped in behind the mercury in the relatively smaller end of the conical bore.

WILLIAM R. JACK.

LYSLE D. CAHILL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,908,908 Loftis May 16, 1938 2,278,846 Goerth Apr. 7, 1942 2,537,671 Jack et al. Jan. 9, 1951 

